The present invention relates to a method for fabricating a semiconductor wafer, and more particularly, to a method for fabricating a semiconductor wafer including a semiconductor crystal layer having strain.
Semiconductor devices using a bulk Si crystal have attained improved multifunctionality and high-speed capability in succession. This attainment is greatly attributed to scale-down of devices. Further device scale-down is required for continuing improvement in device performance in the future. In order to pursue further device scale-down, however, there exist many problems to be technically overcome. If device scale-down proceeds successfully, the optimal performance of the resultant devices is restricted by the physical properties (for example, mobility) of the bulk Si crystal as the material. In other words, as long as the bulk Si crystal is used as the material, it is difficult to dramatically improve the device performance.
In recent years, attempts of using a material other than the bulk Si crystal have been made to improve the device characteristics. One of such attempts is using a new material having a mobility greater than that of Si, such as a mixed crystal of silicon and germanium (SiGe) and a mixed crystal of silicon, germanium, and carbon (SiGeC). Another attempt is using a strained Si crystal. This is an approach of providing a new factor, strain, to a Si crystal to reduce scattering of carrier electrons called intervalley scattering and thus improve the mobility. The latter attempt, in particular, has also received attention from the industrial standpoint, for the reasons that improvement in performance is attained only by giving strain to the bulk Si crystal and that necessary machining of the device can be made only using the existing Si process technology (for example, oxidation and etching process technology).
Conventionally, a strained Si crystal as described above is produced by depositing a thick SiGe crystal layer on a Si substrate made of a bulk Si crystal and then depositing a Si crystal on the SiGe crystal layer. In general, when a SiGe crystal, which has a lattice constant greater than Si, is epitaxially grown on a Si substrate in the state that the lattice in the plane of the substrate is aligned with Si, a considerably large compressible strain is generated in the SiGe crystal. Once the thickness of the SiGe crystal deposited on the Si substrate exceeds a certain thickness (critical thickness), dislocations are generated between the Si substrate and the SiGe layer, and the strain is relieved. As a result, the in-plane lattice constant of the SiGe layer becomes greater than that at the surface of the Si substrate. When a Si crystal layer is epitaxially grown on the SiGe crystal layer, the in-plane lattice constant of the newly deposited Si matches with that of the strain-relieved SiGe crystal, and therefore the Si layer has a lattice constant greater than the inherent lattice constant of Si. As a result, a strained Si crystal layer undergoing tensile stress is produced (hereinafter, a crystal layer that causes lattice relieving and has an interstitial distance greater than a Si substrate, such as the SiGe crystal described above, is called a relieved buffer layer).
A conventional method for forming a strained Si crystal layer on a substrate will be described in more detail with reference to the relevant drawing.
FIG. 1 is a cross-sectional view of a substrate on which a strained Si crystal layer has been formed by a conventional method. To fabricate the substrate including the strained Si crystal layer, first, a SiGe crystal layer 103 having a thickness of several micrometers or more that exceeds a critical thickness is epitaxially grown on a Si substrate 101 by CVD. By this growth, dislocations are generated in the SiGe crystal layer 103, and thus the SiGe crystal layer 103 is subjected to lattice relieving. Thereafter, a Si crystal is deposited on the SiGe crystal layer 103 by CVD, to form a strained Si crystal layer 104.
In the conventional technique described above, a large defect running through the crystal layer (a through dislocation 105) is generated during the formation of the relieved buffer layer made of the SiGe crystal layer 103 having a thickness greater than a critical thickness. The through dislocation 105 may even enter the strained Si crystal layer 104, according to the circumstances, and lead to formation of a defect in the strained Si crystal layer 104. Such a defect in the crystal layer may become a factor impeding improvement of the device characteristics.
To solve the above problem, structures in which the content of Ge in the SiGe crystal layer 103 is changed in stages or in a gradual manner are often used to reduce the density of the through dislocation 105. In any of these structures, however, in order to reduce the density of dislocations, it is necessary to deposit a SiGe crystal layer to a thickness as large as several micrometers while changing the Ge content of the SiGe crystal layer. Long-time crystal growth is required to form such a thick relieved buffer layer, and therefore, cost reduction in wafer fabrication is difficult. For this reason, conventionally, it is considered difficult to use a strained Si crystal for practical fabrication of semiconductor devices.
An object of the present invention is proposing a structure and formation method of a relieved buffer layer having a reduced density of crystal defects, and thereby fabricating a semiconductor wafer including a strained Si layer and the like used as a substrate of a semiconductor device.
The semiconductor wafer of the present invention includes: a substrate made of a Si crystal; and a crystal layer formed on the substrate, the crystal layer having a lattice constant in the plane greater than the lattice constant of the substrate, wherein at least part of the crystal layer is a crystal of Si, Ge, and C with SiC crystals dispersed in the crystal.
With the above construction, the crystal layer having a lattice constant in the plane greater than the lattice constant of the substrate made of Si crystal can be used as the relieved buffer layer. Therefore, a strained Si crystal layer can be formed on the relieved buffer layer. The semiconductor wafer with the above construction can be used as a substrate of a semiconductor device
The semiconductor wafer described above further includes a strained Si crystal layer formed on the crystal layer. When this semiconductor wafer is used as a substrate of a semiconductor device, since the carrier mobility in the strained Si crystal layer is greater than the carrier mobility in a bulk Si crystal, the resultant semiconductor device can exhibit improved performance compared with a semiconductor device using a bulk Si crystal as the substrate.
The first method for fabricating a semiconductor wafer of the present invention includes the steps of: (a) depositing a crystal layer on a substrate made of a Si crystal, at least part of the crystal layer containing Si, Ge, and C; and (b) annealing the substrate including the deposited crystal layer to relieve the lattice of the crystal layer and precipitate SiC crystals in the crystal layer.
By the above method, it is possible to fabricate a semiconductor wafer that uses the crystal layer containing Si, Ge, and C as the relieved buffer layer and enables formation of a strained Si crystal layer hardly having dislocations at positions on the relieved buffer layer.
In particular, in step (b), by annealing the substrate to precipitate SiC, it is possible to suppress generation of a through dislocation in the crystal layer as the relieved buffer layer. It is also possible to reduce the thickness of the relieved buffer layer compared with the conventional relieved buffer layer requiring to be as thick as about several micrometers. This enables mass-production of the semiconductor wafer on which a strained Si crystal layer can be formed.
The first method described above further includes the step of: (c) forming a strained Si crystal layer on the annealed crystal layer including the SiC crystals. By this method, it is possible to fabricate a semiconductor wafer including a relieved buffer layer containing Si, Ge, and C and a strained Si crystal layer. By using this semiconductor wafer as a substrate of a semiconductor device, the semiconductor device can exhibit improved performance compared with a semiconductor device using a bulk Si crystal as the substrate.
The second method for fabricating a semiconductor wafer of the present invention includes the steps of: (a) depositing a crystal layer on a substrate made of a Si crystal, at least part of the crystal layer containing Si, Ge, and C; (b) depositing a Si crystal layer on the crystal layer; and (c) annealing the substrate to precipitate SiC crystals in the crystal layer and strain the Si crystal layer.
By the above method, as in the first method described above, it is possible to fabricate a semiconductor wafer including a relieved buffer layer containing Si, Ge, and c and a strained Si crystal layer. By using this semiconductor wafer as a substrate of a semiconductor device, the semiconductor device can exhibit improved performance compared with a semiconductor device using a bulk Si crystal as the substrate.